Nonlocal Kondo effect and quantum critical phase in heavy fermion metals

TL;DR

This paper introduces a nonlocal Kondo effect in heavy fermion metals, revealing a quantum critical phase with deconfined spinons and explaining experimental puzzles related to Fermi surfaces and metallic spin liquids.

Contribution

It develops a new algorithm for the Schwinger boson approach to incorporate spatial correlations, unveiling a nonlocal Kondo effect and a comprehensive phase diagram for heavy fermion quantum criticality.

Findings

Prediction of a non-Fermi liquid quantum critical phase with a hidden holon Fermi surface.

Explanation of the metallic spin liquid in CePdAl.

Resolution of the Fermi volume puzzle in YbRh₂Si₂.

Abstract

Heavy fermion metals typically exhibit unconventional quantum critical point or quantum critical phase at zero temperature due to competition of Kondo effect and magnetism. Previous theories were often based on certain local type of assumptions and a fully consistent explanation of experiments has not been achieved. Here we develop an efficient algorithm for the Schwinger boson approach to explore the effect of spatial correlations on the Kondo lattice and introduce the concept of nonlocal Kondo effect in the quantum critical region with deconfined spinons. We predict a global phase diagram containing a non-Fermi liquid quantum critical phase with a hidden holon Fermi surface and a partially enlarged electron Fermi surface for strong quantum fluctuations while a single quantum critical point for weak quantum fluctuations. This explains the unusual metallic spin liquid recently reported in the frustrated Kondo lattice CePdAl and resolves the Fermi volume puzzle in YbRh$_2$Si$_2$. Our theory highlights the importance of nonlocal physics and provides a unified understanding of heavy fermion quantum criticality.